Might want to post in the SW
section next time to get more replies. Anyway, it sounds like ich which is a parasite and either your fish that got introduced in the tank had it or it can be caused by stress/bad water conditions. Try quarantining it or if you're not getting corals or inverts, using a copper based medication to treat it. Corals and inverts do not tolerate copper and if you have that in your system they will die, so treat them in a separate tank if you are planning on keeping those animals (kind of redundant, sorry).